The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known spark-ignition (SI) engines introduce an air/fuel mixture into each cylinder which is compressed in a compression stroke and ignited by a spark plug. Known compression ignition engines inject pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke which ignites upon injection. Combustion for both gasoline engines and diesel engines involves premixed or diffusion flames controlled by fluid mechanics.
SI engines can operate in a variety of different combustion modes, including a homogeneous spark-ignition (SI-H) combustion mode and a stratified-charge spark-ignition (SC-SI) combustion mode. When operating in the homogeneous spark-ignition (SI-H) combustion mode, an engine intake valve for each cylinder opens after TDC and a corresponding exhaust valve closes after TDC, creating a positive valve overlap (PVO) period in which both the exhaust and intake valves are open.
SI engines can be configured to operate in a homogeneous-charge compression-ignition (HCCI) combustion mode, also referred to interchangeably as controlled auto-ignition (HCCI) combustion, under predetermined speed/load operating conditions. The controlled auto-ignition (HCCI) combustion comprises a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry. An engine operating in the controlled auto-ignition (HCCI) combustion mode has a cylinder charge that is preferably homogeneous in composition, temperature, and residual exhaust gases at intake valve closing time. Controlled auto-ignition (HCCI) combustion is a distributed kinetically-controlled combustion process with the engine operating at a dilute air/fuel mixture, i.e., lean of an air/fuel stoichiometric point, with relatively low peak combustion temperatures, resulting in low nitrous oxides (NOx) emissions. The homogeneous air/fuel mixture minimizes occurrences of rich zones that form smoke and particulate emissions.
Controlled auto-ignition (HCCI) combustion depends upon factors including cylinder charge composition, temperature, and pressure at intake valve closing. The control inputs to the engine are coordinated to facilitate robust auto-ignition combustion. Controlled auto-ignition (HCCI) combustion strategies may include using an exhaust recompression valve strategy. The exhaust recompression valve strategy includes adjusting timing of the intake and exhaust valves relative to TDC to control a cylinder charge temperature by trapping residual gas from a previous engine cycle. In operation, the exhaust valve closes before TDC and the corresponding intake valve opens after TDC creating a negative valve overlap (NVO) period in which both the exhaust and intake valves are closed, thereby trapping the exhaust gas. The opening timings of the intake and exhaust valves are preferably symmetrical relative to TDC. Both a cylinder charge composition and temperature are affected by the exhaust valve closing timing. In particular, more hot residual gas from a previous cycle can be retained with earlier closing of the exhaust valve leaving less room for incoming fresh air mass, thereby increasing cylinder charge temperature and decreasing cylinder oxygen concentration.
Different combustion modes in similar speed/load situations can have performance differences relating to engine stability, emissions, and fuel economy. Engine operation can include transitioning to a particular combustion mode having a preferred performance in a particular situation. Selecting a preferred combustion mode in which to operate can be based upon which combustion mode results in preferred performance at a particular engine load and speed. When a change in speed and/or engine load warrants a transition to a different combustion mode, a transition strategy will be performed and the engine will transition to the different combustion mode.
Known challenges associated with transitioning engine operation between combustion modes include incomplete combustion, engine misfires, torque disturbances, and increased undesirable emissions.